This procedure provides a technique for conducting electrochemical experiments on precise microstructural features on a material surface. This procedure furthermore minimizes loss of signal due to high solution resistance.
Earlier procedures suffer from several disadvantages. Examples include microcapillaries and lacquers. Microcapillaries have the disadvantages of high solution resistance through the capillary, solution leakage at the seal with the sample surface, and imprecise capillary placement.
Using lacquers is a technique that was suggested in the 1970s and consisted of coating the surface to be sampled with a lacquer and then to make small pinholes over the regions to be investigated. Again, the use of lacquers as a technique suffers from several disadvantages.
One of the challenges associated with electrochemical testing is that it is difficult to determine individual contributions to a measured current especially when the sample area comprises multiple grains, grain boundaries, precipitates, etc. The heterogeneity of such areas result in competing kinetic processes that contribute to the overall current.
For studies that are aimed at determination for example, of corrosion resistance or catalytic activity, it would be beneficial to have a versatile technique that can isolate areas of interest.
Localized experimental procedures using micro-capillaries (1-3) to probe small areas of the sample surfaces have been developed previously. Though the micro-capillary technique has been in widespread use for the past 15 years, aspects of the technique make it undesirable for certain experimental procedures. For example, the relatively fast potentiodynamic sweep rates required to prevent cell leakage or tip blockage of the micro-capillary prevent scanning at rates as slow as 10 mV/min and the micro-capillary tip diameter can affect the limiting current passing through the cell. In addition a flat, polished surface is needed for this technique.
Here, a technique for making electrochemical measurements on isolated individual phase regions of known crystalline orientation in a duplex stainless steel is demonstrated. An ultraviolet-sensitive photoresist is used to mask the excluded portions of the sample and a 355 nm laser exposes only portions of the ferrite matrix or cross-sections of austenite dendrites. Initial impedance measurements indicate a relatively low solution resistance in seawater and the polarization scans of the ferrite and austenite phases were consistent with bulk polarization measurements.